1. Field of the Invention
The present invention relates to a mobile communication system, a base station in a mobile communication system, a radio network control apparatus, a mobile station and a power consumption decreasing method, in which, the mobile station omits receiving level measurement for a signal from base stations when a signal energy to noise power density ratio of a common pilot signal (to be referred to as Ec/No hereinafter) exceeds a predetermined threshold, wherein the common pilot signal is sent to the mobile station from a base station that forms a cell where the mobile station resides.
2. Description of the Related Art
In W-CDMA (Wide-band Code Division Multiple Access), which is a radio access method in the third generation mobile communication system, when a mobile station does not reside in an optimal cell among cells formed by base stations while the mobile station is in a standby status, a control signal of upward direction (called an uplink control signal hereinafter) sent from the mobile station may not reach the base station, or even when the signal reaches the base station, the uplink control signal may cause excessive interference for cells formed by other base stations, so that system capacity may be degraded. In addition, it becomes hard for the mobile station to receive a control signal of downward direction (called a downlink control signal hereinafter) sent from the base station, so that the base station needs to send the downlink control signal by using a large sending power. The system capacity is degraded also for this reason. Therefore, in W-CDMA, it is necessary for the mobile station to reside in an optimal cell while the mobile station is in a standby status.
In order for the mobile station to reside in an optimal cell, the mobile station needs to search signals sent from surrounding base stations and to measure receiving levels of the signals (this measurement is called “surrounding cell measurement” hereinafter). However, since the surrounding cell measurement consumes much power, if the mobile station performs the surrounding cell measurement simply periodically without any remedy, battery life of the mobile station becomes short. Thus, in the W-CDMA, a method for performing surrounding cell measurement only when necessary is adopted. In the method, the mobile station measures Ec/No per one chip of the common pilot signal sent from a base station forming a cell where the mobile station resides (to be referred to as a residing cell hereinafter), and if the measured value exceeds a predetermined threshold (which is unchangeable), it is determined that the residing cell is optimal, so that surrounding cell measurement is omitted.
However, if the threshold is a fixed value, a following problem described with reference to FIG. 1 arises.
A case is considered in which a small constant value is set as the threshold of Ec/No assuming that total sending power of signals sent from base stations (to be referred to as downlink total sending power hereinafter) is large. The reason for setting a small constant value as the threshold of Ec/No is that, if the downlink total sending power is large, interference power of downward direction (called downlink interference power) becomes large, so that Ec/No becomes small.
In FIG. 1, region 1 is a region of a cell formed by a base station, and region 2 indicates a region where the surrounding cell measurement is unnecessary, and threshold A indicates a fixed threshold of Ec/No established considering a case when the downlink total sending power is large.
As shown in FIG. 1, when the downlink total sending power is actually large, Ec/No in the mobile station becomes less than the threshold A in a position where the mobile station is out of the region 2. Therefore, the surrounding cell measurement can be performed only when necessary if the surrounding cell measurement is performed only when Ec/No becomes smaller than the threshold A.
However, when the total sending power is actually small, Ec/No in the mobile station does not become less than the threshold A until the mobile station goes out of the region 3 which is larger than the region 2. Therefore, if the surrounding cell measurement is performed only when Ec/No becomes less than the threshold A, there occurs a problem in that the surrounding cell measurement is not performed even when the mobile station resides in a region (between the region 2 and the region 3) where the surrounding cell measurement is necessary.
Next, with reference to FIG. 2, a case is considered in which a large constant value is set as the threshold of Ec/No assuming that downlink total sending power is small, for realizing the method in which surrounding cell measurement is performed only when necessary. The reason for setting a large constant value as the threshold of Ec/No is that, if the downlink total sending power is small, downlink interference power becomes small, so that Ec/Nc becomes large.
In FIG. 2, region 1 is a region of a cell formed by a base station, and region 2 indicates a region where the surrounding cell measurement is unnecessary, and threshold B indicates the threshold of Ec/No established assuming that the downlink total sending power is small.
As shown in FIG. 2, when the total sending power is actually small, Ec/No in the mobile station becomes less than the threshold value B in a position where the mobile station is out of the region 2. Therefore, the surrounding cell measurement is to be performed as necessary if the surrounding cell measurement is performed only when Ec/No becomes smaller than the threshold B.
However, if the total sending power is actually large in this case, the Ec/No in the mobile station becomes less than the threshold B if the mobile station goes out of the region 3, which is smaller than the region 2. Therefore, when the surrounding cell measurement is to be performed only when Ec/No becomes less than the threshold B, there occurs a problem in that the measurement is performed even when the mobile station resides in a region (between the region 2 and the region 3) where the surrounding cell measurement is unnecessary.
In addition, even when downlink total sending power is constant, Ec/No differs according to the size of the cell and to a condition for which the base station is placed such as in an urban area or in a suburban area. Therefore, when the threshold is set suitable for the urban area for surrounding cell measurement to be performed as necessary, there is a problem in that the measurement is not performed even when a mobile station resides in a region where the measurement is necessary in a suburban area, or, on the other hand, measurement is performed even when the mobile station is not in a region where the measurement is necessary.
Thus, a method is required for decreasing power consumption of a mobile station by performing surrounding cell measurement only when necessary, in other words, a method of improving battery life is required.